JP4397963B2 - Lead-free brass alloy with excellent stress corrosion cracking resistance - Google Patents

Description

  The present invention relates to a leadless brass alloy containing Bi and excellent in stress corrosion cracking resistance, and more particularly to a leadless brass alloy that improves the stress corrosion cracking resistance by suppressing the occurrence of corrosion cracking in the brass alloy. .

Generally, brass alloys such as JIS CAC203, C3604, and C3771 are excellent in properties such as corrosion resistance, machinability, and mechanical properties, and thus are widely used in plumbing equipment for water supplies such as valves, cocks, and joints, and electronic equipment parts. ing.
This type of brass alloy may cause stress corrosion cracking particularly when it is exposed to a corrosive environment such as an ammonia atmosphere and a tensile stress is applied. Conventionally, various proposals have been made as measures for preventing stress corrosion cracking in this brass alloy.

  For example, the brass material of Patent Document 1 contains Cu: 57 to 61%, Pb: 1 to 3.7%, Sn content is 0.35% or less, and is composed of two phases of α + β at room temperature. Brass with an average crystal grain size of α of 15 μm or less, an average crystal grain size of β phase of 10 μm or less, and a phase ratio of α phase exceeding 80% to improve the stress corrosion cracking resistance. is there.

  Patent Document 2 has a crystal structure of α + β + γ at normal temperature, the area ratio of α phase is 40 to 94% at normal temperature, the area ratio of β phase and γ phase is both 3 to 30%, and the average of α phase and β phase Brass has been proposed in which the crystal grain size is 15 μm or less, the average crystal grain minor axis of the γ phase is 8 μm or less, the γ phase contains 8% or more of Sn, and the β phase is surrounded by the γ phase. This brass is also brass intended to improve the resistance to stress corrosion cracking by containing high Sn, and also contains 1.5 to 2.4 wt% of Pb.

JP 2006-9053 A Japanese Patent No. 3303301

    However, the brass material of Patent Document 1 is applied particularly as a material for flare nuts, and is not suitable for a material for plumbing equipment for waterworks. That is, this brass contains a large amount of Pb, and such Pb-containing brass has an adverse effect on the human body and cannot be applied to plumbing equipment for water supply.

By the way, the present inventors conducted a test according to the conditions under which stress corrosion cracking occurs, and as a result of observing the cracking forms of the conventional Bi-based leadless brass alloy in which stress corrosion cracking occurred and lead-containing brass alloy, In the form of stress corrosion cracking, it has been clarified that lead-containing brass is caused by branching of fine cracks, whereas Bi-based leadless brass is relatively large in cracks (FIG. 1 ( a), see FIG.

When cracks caused by stress corrosion cracking are compared between lead-containing copper alloys and lead-less copper alloys, the cracks in lead-containing brass alloys are such that fine cracks branch into a large number, as shown in FIG. This branching crack tends to make it difficult for further cracks to progress and make the cracks shallower.
On the other hand, the crack of the lead-less brass alloy (for example, Bi-based lead-less brass alloy) is a relatively large crack as shown in FIG. 1A, and the crack progresses deeply due to this one crack. We confirmed the phenomenon of the trend.

This is because lead-containing copper alloys are prone to branch when the tip of the stress corrosion crack contacts the slip band (the surface on which metal atoms slide during metal deformation), and stress is dispersed by this branch. On the other hand, Bi-based leadless copper alloys are unlikely to branch at the slip band, and it is considered that stress concentration is likely to occur due to linear cracking.
For this reason, in particular, Bi-based leadless copper alloys require measures for cracking that are different from those of lead-containing brass alloys, and specifically, the progress of cracking due to stress concentration caused by the occurrence of linear cracking. It is necessary to take measures in terms of materials that prevent

  Based on the above observation results, the problems in Patent Document 2 are described. As described in the same document (Examples), this brass alloy is all Pb-added and lead-free. The fact that it can also be used for brass alloys is not described actively.

In the α + γ type and α + β + γ type in Patent Document 2, there is a description that the stress corrosion cracking resistance has been improved by using the γ phase. In particular, the area ratio, composition, and size of the γ phase are quantitatively determined. Explanation is made. However, when cracks progress linearly without branching like lead-free copper alloys, the most important issue is how the γ phase is distributed with respect to the direction of crack propagation. This point is not described, and it is insufficient as a countermeasure against stress corrosion cracking. In other words, this technology is a technology for specifying the γ phase by an absolute amount such as an area ratio, and there is no suggestion of a matter or technical idea of stopping the linear cracks peculiar to leadless brass by dispersing the γ phase. .
Based on this technique, by increasing the Sn content, it is conceivable to surround the crystal grains with the γ phase and increase the absolute amount of the γ phase in the crack progressing direction. There is a possibility that a new problem may occur that casting defects such as a nest may occur.

Moreover, although the copper alloy of patent document 2 precipitates the γ phase by containing a large amount of Sn and attempts to improve the stress corrosion cracking resistance by this γ phase, the document 2 discloses that Pb is contained. Since a large amount of Sn was added to the contained brass, as shown below, a decrease in stress corrosion cracking property was confirmed.
That is, the brass used for the test here is the test materials a to h having the chemical composition values shown in Table 1 according to the die casting product, and the test method is 9 for the Rc1 / 2 threaded portion of each test material a to h. Screw a stainless steel bushing with a torque of 8 N · m (100 kgf · cm), expose to a 14% ammonia atmosphere, and visually observe the presence or absence of cracks in each specimen up to a maximum of 48 hours at a predetermined elapsed time. And judged. An example of the test material at this time is shown in FIG. 2 and a schematic diagram of a test apparatus used in the stress corrosion cracking test is shown in FIG. Table 1 shows the chemical composition values of each specimen and the results of stress corrosion cracking (depending on the stress corrosion cracking time), and FIG. 48 shows the time until stress corrosion cracking with respect to the Sn content of each specimen. Indicated. In addition, about a test method, it demonstrates in the evaluation criteria of the stress corrosion cracking resistance mentioned later.

  As a result, it was found that the stress corrosion cracking time was shortened and the stress corrosion cracking resistance was lowered with an increase in the Sn content. Accordingly, the document 2 cannot be said to be a technology that can be used as it is for a lead-less brass alloy as long as the stress corrosion cracking resistance is not necessarily improved with respect to Pb-containing brass.

In view of the above-mentioned problems, the present invention has been developed as a result of earnest research, and its purpose is to improve the stress corrosion cracking resistance in leadless brass alloys. by inhibiting the rate of progression of corrosion cracking in the brass alloy, halt the leadless brass alloys peculiar linear crack, a high order of probability of cracking γ phase present at the grain boundaries is in contact, with, resistant An object of the present invention is to provide a leadless brass alloy that can contribute to improvement of stress corrosion cracking property.

In order to achieve the above object, the invention according to claim 1 is based on a mass ratio of Cu 59.5 to 66.0%, Sn 0.7 to 2.5%, Bi 0.5 to 2.5%, Sb 0.05. It is a brass alloy having an α + γ structure containing the Zn and inevitable impurities, or an α + β + γ structure with a balance of ~ 0.6%, and solid solution of Sb in the brass alloy component in the γ phase, and in the brass alloy The ratio of the γ phase to each crystal grain when the γ phase surrounds each crystal grain is defined as the γ phase grain surround ratio, and the average value of the γ phase grain surround ratio is 28%. By setting it as the above, it is the leadless brass alloy excellent in the stress corrosion cracking resistance which suppressed the progress rate of the corrosion cracking in a brass alloy, and improved the stress corrosion cracking resistance.

The invention according to claim 2, Se: 0.01 to 0.20 is a leadless brass alloy which is excellent in the stress corrosion cracking resistance containing mass%.

According to the first aspect of the present invention, the progress of the corrosion cracking in the brass alloy is suppressed, thereby delaying the progress of the linear crack peculiar to the lead-less brass alloy, thereby improving the resistance to stress corrosion cracking. A less brass alloy can be provided. In this case, by dissolving Sb in the γ phase, a brass alloy excellent in stress corrosion cracking resistance that can secure stress corrosion cracking resistance equal to or higher than that of lead-containing brass alloys such as hexagonal brass containing lead. Can be obtained.

In addition , by setting the average crystal surrounding ratio of the γ phase existing in the grain boundary to 28% or more, cracks are not generated when the stress load direction is not specified, that is, when the progress direction of the crack is not specified. By increasing the probability of contact with the γ phase and slowing the rate of progress of corrosion cracking, it is possible to stop the cracks peculiar to Bi-containing leadless brass alloys, and thus the stress resistance in Bi-containing leadless brass alloys. It has become possible to provide a brass alloy capable of improving the corrosion cracking property.

According to the invention which concerns on Claim 2 , in addition to containing Sn, stress corrosion cracking resistance can be further improved by containing Se .

It is the enlarged photograph which showed the crack state of the brass alloy. (A) is an enlarged photograph showing a typical cracked state of a Bi-based leadless brass alloy. (B) is an enlarged photograph showing a typical cracked state of a lead-containing brass alloy. It is an external view of a test material. It is the schematic diagram which showed the test apparatus used for the stress corrosion cracking test. It is the graph which showed the result of the stress corrosion cracking time of the specimen used for evaluation criteria. It is explanatory drawing which showed the manufacturing method of the bar manufactured from the billet of a brass alloy. It is the enlarged photograph which showed the microstructure of the bar. It is the graph which showed the relationship of (gamma) phase average grain surrounding rate of the brass alloy of this invention, and stress corrosion cracking time. It is the graph which showed the relationship between the measurement number of (gamma) phase surrounding ratio, and the crystal grain surrounding ratio of (gamma) phase. It is explanatory drawing which showed the measurement location of the test material. (A) is the schematic diagram which showed the measurement location in a test material. (B) is the A section enlarged view. It is the graph which showed the relationship between the number of (gamma) phase contacts, and stress corrosion cracking time. It is an enlarged photograph which showed the measurement state of the contact number of the (gamma) phase in the predetermined location of a test material. It is explanatory drawing which showed the measurement state of the contact number of the (gamma) phase in the predetermined location of a test material. It is explanatory drawing which showed the measurement state of the contact number of the (gamma) phase in the other location of a test material. It is explanatory drawing which showed the area | region of the average value-standard deviation in a normal distribution map with the oblique line. It is the bar graph which showed the relationship between Sn content of the test material of the brass alloy of this invention, and stress corrosion cracking time. It is the bar graph which showed the relationship between Sb content of the test material of the brass alloy of this invention, and stress corrosion cracking time. It is the line graph which showed the relationship between Sb content of the test material of the brass alloy of this invention, and stress corrosion cracking time. It is the enlarged photograph which showed the EMPA mapping analysis result of the specimen 3 ((alpha) + (beta) + (gamma) structure | tissue). (A) is the enlarged photograph which showed the measurement result by SEM-EDX of the test material 3 ((alpha) + (beta) + (gamma) structure | tissue). (B) is explanatory drawing which showed the composition in the analysis location of a number. It is an enlarged photograph which showed the EMPA mapping analysis result of the specimen 4 ((alpha) + (gamma) structure | tissue). (A) is the enlarged photograph which showed the measurement result by SEM-EDX of the specimen 4 ((alpha) + (gamma) structure | tissue). (B) is explanatory drawing which showed the composition in the analysis location of a number. It is the line graph which showed the relationship between Cu content of the test material of the brass alloy of this invention, and stress corrosion cracking time. It is the schematic which showed the external appearance of the test material, and the measurement location of stress. It is the graph which showed the Bi content of the test material of the brass alloy of this invention, and the relationship of stress. It is the schematic explanatory drawing which showed the clearance gap jet test apparatus. It is a phase diagram of 1% Sn containing brass. It is the graph which showed the relationship between an evaluation coefficient and stress corrosion cracking time. It is the enlarged photograph which showed the distribution state of (gamma) phase. It is a graph at the time of changing the reference value of bar diameter (φ1). It is the graph which showed the relationship between pregelatinization temperature and the fracture time of stress corrosion cracking property. It is the graph which showed the change by the influence degree (0.6) of drawing. It is the graph which showed the change by the influence degree (0.4) of drawing. It is the graph which showed the change by the influence degree (0.2) of drawing. It is the schematic sectional drawing which showed the corrosion state of the metal. (A) is sectional drawing which showed the state of the whole surface corrosion. (B) is sectional drawing which showed the state of local corrosion. It is a schematic plan view showing the length and width of the α phase of the alloy. It is explanatory drawing which showed the tension | pulling direction and observation surface in a tension | pulling SCC property test. It is the graph which showed the relationship between the structure | tissue parameter and the fracture | rupture time at the time of a tensile SCC property test. It is the graph which showed the relationship between corrosion time and the maximum corrosion depth / average corrosion depth. It is the graph which showed the relationship between corrosion time and a variation coefficient. It is a microstructure cross-sectional photograph before and behind the corrosion test of the brass material of this invention and a comparative example. It is the photograph which showed the surface layer structure before the corrosion of the brass material of this invention and a comparative example. It is the photograph which showed the surface layer structure | tissue after corrosion of the brass material of this invention and a comparative example. It is an enlarged photograph of a microstructure cross-sectional photograph . It is the graph which showed the relationship between corrosion time and average corrosion depth. It is the graph which showed the relationship between corrosion time and the maximum corrosion depth. It is the schematic of a tensile test piece. (A) is a top view of a tensile test piece. (B) is a front view of a tensile test piece. It is the graph which showed the relationship between the load stress and the fracture | rupture time in a tension test. It is a graph which shows the relationship between Sn content at the time of the SCC property test of a Pb containing brass alloy, and the time until a crack arises. It is the graph which showed the amount of Sn of Bi type, Bi-Se type casting, and the relationship of SCC property.

  A preferred embodiment of the leadless brass alloy in the first invention will be described. The Bi-containing leadless brass alloy shown in FIG. 1 (a) is linear in corrosion cracking and, as will be described in detail below, by suppressing the rate of progress of the corrosion cracking as much as possible, the stress corrosion cracking resistance is improved. It was possible to improve.

  The brass alloy according to the first invention forms an α + γ structure or an α + β + γ structure by containing Sn in a Bi-containing leadless brass alloy (especially hexagonal brass), and is precipitated in this structure. Distributing the γ phase on the basis of a certain law causes excellent stress corrosion cracking resistance.

  As a constant law of the γ phase at this time, in this brass alloy, the ratio of the γ phase to each crystal grain when the γ phase surrounds each crystal grain in the alloy structure is defined as the γ phase grain enclosure rate. Then, the average value of the γ phase crystal grain sieving rate is defined as the γ phase average crystal grain sieving rate, and in the examples, a correlation between the γ phase average crystal grain sieving rate and the stress corrosion cracking resistance is derived. When the average grain surrounding ratio when the predetermined stress corrosion cracking time could be satisfied was confirmed, the γ phase average grain surrounding ratio was 28% or more. As a result, it was derived that the γ-phase average grain surrounding ratio in this brass alloy was 28% or more.

In addition, as another regular rule of the γ phase, in the brass alloy in the first invention, when a stress load is applied to the alloy, stress corrosion cracking occurs, and assuming the γ phase that the crack contacts, The number of γ phases existing in the unit length in the vertical direction is defined as the number of contacts of the γ phase, and the numerical value calculated from the average value and standard deviation of the number of contacts is defined as the number of contacts of the γ phase. The correlation between the number of contacts and the stress corrosion cracking time is derived, and when the number of γ phase contacts when the predetermined stress corrosion cracking time can be satisfied is confirmed based on this correlation, the number of γ phase contacts is two or more. It became. Thereby, it was derived that the number of γ-phase contacts in this brass alloy was 2 or more.

  Therefore, a detailed definition of the γ-phase average crystal grain enclosure ratio and the number of γ-phase contacts in this copper alloy and examples carried out for deriving these numerical values will be described. Explains the evaluation criteria brass alloy necessary to compare the performance of leadless brass alloy and stress corrosion cracking resistance, the elements contained in this brass alloy and their composition range, etc., and stress corrosion resistance that this brass alloy can exhibit The crackability will be described.

(Evaluation criteria for stress corrosion cracking resistance)
In describing the stress corrosion cracking resistance that the brass alloy of the first invention can exhibit, an evaluation standard for comparing its performance is required. Therefore, first, this evaluation standard is set using five types of lead-containing hexagonal brass bars that are generally widely used and have few problems of stress corrosion cracking resistance.

  As a stress corrosion cracking test method in the present embodiment, 9.8 N · m (100 kgf · 100 mf) is applied to each of the specimens a to e as shown in FIG. A stainless steel bushing (hollow male screw part) is screwed in with a torque of cm) and exposed to a 14% ammonia atmosphere at a predetermined elapsed time (4, 8, 12, 24, 36, 48 hr) up to a maximum test time of 48 hr. Every time, each test material was taken out from the desiccator and washed, and then a test for judging the presence or absence of cracking of each test material by visual confirmation was performed. Specifically, as shown in FIG. 3, 2 L of ammonia water having a concentration of 14% is accommodated at the bottom of a desiccator that accommodates an intermediate plate having an outer diameter of 300 mm, and a cylindrical specimen is disposed on the upper surface of the intermediate plate. did. This test material is placed inside the desiccator so that the hollow bushing is screwed up and the ammonia gas comes into contact with the inside of the test material through a vent hole in the middle plate. Housed in. The distance t between the upper surface of the ammonia water and the intermediate plate is about 100 mm, and the test material is in non-contact with the ammonia water.

Here, it is known that the stress corrosion cracking is generally caused by the simultaneous action of three factors of material factor, environmental factor, and stress factor, and the mechanism is complicated. Therefore, when conducting stress corrosion cracking tests, the effects of materials, processing, stress loading, test environment, etc. may cause variations in the test results, so be careful to keep the test conditions as identical as possible. And tested.
Table 2 shows the chemical components (mass%) of the hexagonal brass bars (up to specimens i to m) used for the evaluation criteria and the stress corrosion cracking time (hr) in each specimen.

    This test was performed with a maximum test time of 48 hours, and FIG. 4 shows a graph of the results of each stress corrosion cracking time obtained from Table 2. The shortest stress corrosion cracking time was 12 hr in the test material j and the test material l, but the actual products manufactured with substantially the same components as those of the test material 1 Since almost no occurrence occurred, 12 hr was adopted as the standard B in the present invention. Further, as a more preferable criterion A, 26 hr, which is an average value of the test materials im, was employed.

Here, the elements contained in the Bi-containing lead-less brass alloy in the first invention, the desirable composition range thereof, and the reason thereof will be described.
As described above, the crack form of the lead-containing brass alloy by stress corrosion cracking is divided into a large number of fine cracks, and the cracks do not proceed any further. On the other hand, in a leadless brass alloy, one relatively large crack proceeds deeply due to stress concentration. That is, the conventional lead-containing brass alloy and the lead-less brass alloy have fundamentally different cracking forms due to stress corrosion cracking as shown in FIGS. 1 (a) and 1 (b). Measures to delay the progress of cracking are essential for the stress corrosion cracking.

Sn: 0.7-2.5 mass%
Sn is well known as an element for improving dezincification corrosion resistance and erosion / corrosion resistance in brass alloys, but in the first invention, it is mainly contained as an element contributing to the above-described improvement in stress corrosion cracking resistance. It is an essential element. By containing Sn, the γ phase is precipitated, and the γ phase is distributed in the alloy structure based on the law described in detail later, thereby suppressing the progress of stress corrosion cracking of the alloy.

As the Sn content, 0.7 mass% or more is effective as described above in order to satisfy the above-mentioned standard B (12 h) of stress corrosion cracking resistance, and further to satisfy the standard A (26 h). 1.0 mass% or more (more surely, 1.1 mass% or more) is effective.
On the other hand, when Sn is contained excessively, defects (snail) are generated inside the cast product. Therefore, in order to obtain stress corrosion cracking resistance satisfying the standard A while suppressing the content, the content is set to 2.5 mass% or less. Is preferred. Further, if Sn is contained excessively, the machinability is lowered or the mechanical properties (particularly elongation) is lowered, so the content is preferably 2.0 mass% or less.

Sb: 0.05-0.60 mass%
Sb is an element that improves the dezincing resistance of the brass alloy. In the first invention, Sb is contained in addition to the inclusion of Sn when further improving the stress corrosion cracking resistance. In the case of a brass alloy containing Sn-containing Bi + Sb or Sn-containing Bi + Se + Sb and having an α + γ structure or an α + β + γ structure, it is an essential element. In other cases, it is an optional element. In the initial stage of corrosion, the surface layer containing the γ phase in which Sb is dissolved dissolves in the form of full-surface corrosion, and therefore it is possible to suppress the occurrence of cracks that are the starting point of stress corrosion cracking. Further, Sb is dissolved in the γ phase, and the hardness of the γ phase is increased, so that the progress of the crack can be suppressed even when the crack occurs.

In order to improve the stress corrosion cracking resistance by the inclusion of Sb, 0.05 mass% or more (more surely, 0.06 mass% or more) on the premise of containing Sn: 0.7 to 2.5 mass% Containing is effective.
On the other hand, if Sb is contained excessively, the stress corrosion cracking resistance is lowered, so that 0.60 mass% (more) is required to obtain the stress corrosion cracking resistance satisfying the standard B (12h) while suppressing the content. To be sure, the upper limit should be 0.52 mass%. Moreover, 0.06-0.21 mass% is optimal as content of Sb which can obtain | require standard A (26h) reliably.
When considering dezincing resistance, the maximum dezincing depth of ISO is suppressed to 10 μm or less by containing 0.08 mass% of Sb, and the suppression effect is saturated by the addition of more Sb. The optimum content of Sb that satisfies the dezincing property and the stress corrosion resistance cracking property (standard A) and is suppressed to the necessary minimum is around 0.08 to 0.12 mass%.

Cu: 59.5-66.0 mass%
Cu is an essential element that needs to be contained in an amount of 59.5 mass% or more on the premise that a γ phase is precipitated by the inclusion of Sn and an alloy having an α + γ structure or an α + β + γ structure is obtained. In order to satisfy the above-mentioned stress corrosion cracking resistance standard B (12h), it is effective to contain 59.5 mass% or more (more surely, 59.6 mass% or more) of Cu. In order to satisfy the above), it is effective to contain 60.0 mass% or more (more surely, 60.6 mass% or more). On the other hand, if Cu is contained excessively, the stress corrosion cracking resistance is rather lowered, so 66.0 mass% (more certainly, 65.3 mass%) should be made the upper limit.

Bi: 0.5-2.0 mass%
Bi is an essential element contained for improving machinability. In order to obtain machinability equivalent to general leadless brass, a content of 0.5 mass% or more is required. On the other hand, when Bi is contained excessively, the tensile strength and elongation are lowered, so the content is preferably 2.0 mass% or less.
As a factor of stress corrosion cracking, which is the subject of the present invention, there is residual stress in the alloy after cutting, and a technology for suppressing stress corrosion cracking is known by converting this residual stress from tensile stress to compressive stress. It has been. As a result of forming the above-mentioned specimen (Rc1 / 2 threaded part) by cutting and measuring the residual stress, the residual stress can be made a compressive stress by containing Bi by 0.7 mass% or more. Therefore, when stress stress cracking resistance is important, the Bi content is preferably 0.7 to 2.0 mass%.

Se: 0.00-0.20 mass%
Se is an optional element contained when ZnSe and CuSe exist in the alloy, and this improves the machinability by acting as a chip breaker. In order to obtain machinability equivalent to that of general leadless brass, it is effective to contain Se together with Bi, and more certainly to contain 0.01 mass% or more. At this time, the machinability is improved as the Se content is increased, but if it is excessively contained, the tensile strength is reduced, so the content is 0.20 mass% or less.
Moreover, according to the Example mentioned later, since the stress corrosion cracking resistance improves by containing Se in addition to the inclusion of Sn, Se is an essential component to further improve the stress corrosion cracking resistance. It is an element. However, even if it is contained excessively, its action reaches a peak, so the upper limit when stress stress cracking resistance is emphasized is 0.09 mass%. In addition, even if Se is contained in a small amount (for example, 0.03 mass% or more) by recycling the leadless brass alloy, the stress corrosion cracking resistance is improved.

  Since ZnSe and CuSe, which are intermetallic compounds, are present at the grain boundaries and are hard, the progress of stress corrosion cracking of the alloy can be effectively suppressed together with the γ phase precipitated by the inclusion of Sn.

  As a specific example, using a billet 2 described in Table 3 to be described later, a test material (bar material) is manufactured by the method B described in FIG. 5, and the α phase found in the microstructure of the test material The micro Vickers hardness of the intermetallic compound ZnSe was measured at five locations. The average values were 81 for the α phase and 103 for ZnSe, and it was revealed that ZnSe was harder than the α phase. Therefore, in addition to the γ phase, the progress of cracking can be further suppressed by precipitating a hard intermetallic compound containing Se.

Ni: 0.05-1.5 mass%
Ni is an optional element that is contained when the tensile strength is improved. Although the effect is observed when the content is 0.05% by mass or more, the effect is saturated even if the content is excessively increased. Therefore, the upper limit is 1.5 mass%. Ni is also an element that improves the yield of Se when Se is contained in the alloy. When the yield of Se is improved, the content is 0.1 to 0.3 mass%. preferable.

P: 0.05-0.2 mass%
P is contained as an essential element for improving dezincing resistance in an alloy not containing Sb. P is effective when contained in an amount of 0.05 mass% or more, and the dezincing resistance is improved as the content is increased. However, since the tensile strength is lowered, the upper limit is 0.2 mass%. In the alloy containing Sb, P is an optional element and is contained when the dezincing resistance is further improved.

Inevitable impurities: Fe, Si, Mn
Examples of the inevitable impurities of the brass alloy embodiment in the present invention include Fe, Si, and Mn. When these elements are contained, the machinability of the alloy decreases due to the precipitation of a hard intermetallic compound, and adverse effects such as an increase in the replacement frequency of the cutting tool occur. Therefore, Fe: 0.1 mass% or less, Si: 0.1 mass% or less, and Mn: 0.03 mass% or less are treated as inevitable impurities having low influence on machinability.
In addition, As: 0.1 mass% or less, Al: 0.03 mass% or less, Ti: 0.01 mass% or less, Zr: 0.1 mass% or less, Co: 0.3 mass% or less, Cr: 0.3 mass% or less, Ca: 0.1 mass% or less and B: 0.1 mass% or less are mentioned as inevitable impurities.

Based on the above elements, the Bi-containing leadless brass alloy of the present invention is constituted. The composition of a typical alloy is as follows (the unit of the component range is mass%. Sb and Se may be optional components depending on the purpose).
(Alloy 1: “Alloy satisfying evaluation standard B (12h) for resistance to stress corrosion cracking”)
Sn: 0.7-2.5
Sb: 0.06-0.60
Cu: 59.5-66.0
Bi: 0.5-2.0
Se: 0 <Se ≦ 0.20
The rest: Zn and inevitable impurities

(Alloy 2: “Optimum alloy that satisfies stress corrosion cracking resistance evaluation standard A (26h)”)
Sn: 1.0-2.5
Sb: 0.08 to 0.21
Cu: 60.0-66.0
Bi: 0.7-2.0
Se: 0.03-0.09
The rest: Zn and inevitable impurities

Next, in the brass alloy containing the above-described elements, the relationship with the stress corrosion cracking resistance when the γ phase is distributed in the alloy structure based on a certain law, specifically, the γ phase average grain enclosure rate And the relationship between stress corrosion cracking resistance and the number of γ phase contacts and stress corrosion cracking resistance.
Here, the γ phase in the alloy of the present invention is composed of Cu, Zn, Sn, or Cu, Zn, Sn, Sb as main elements, and α phase and β phase (both main constituent elements are Cu, It precipitates at the grain boundaries of the crystal grains formed by (Zn). Since the γ phase is harder than the α phase, the progress of the crack can be slowed by contacting the tip of the stress corrosion crack that progresses in the alloy structure with the γ phase.
Therefore, by increasing or varying the amount of the γ phase, the probability that the crack contacts the γ phase can be increased, and the stress corrosion cracking resistance of the alloy can be improved.

  Therefore, the amount and variation of the γ phase necessary to improve the stress corrosion cracking resistance of lead-free copper alloys (collectively referred to as “distribution”) are expressed as “γ phase average grain enclosure rate”, “γ It was specified using the index “number of phase contacts”. Hereinafter, the correlation between the detailed definition of “γ phase average crystal grain enclosure ratio” and “γ phase contact number” and the stress corrosion cracking resistance will be described.

First, the Example in the relationship between (gamma) phase average grain surrounding ratio and stress corrosion cracking is explained in full detail.
“Γ phase average crystal grain enclosure rate” means the outer peripheral length of a grain boundary (grain boundary of crystal grains (α phase)) and the length of the γ phase existing on the outer periphery at an arbitrary site in the alloy. Based on the average value of data obtained by measuring and performing a plurality of measurements, the following equation is used.
[Formula 1]
γ-phase average crystal grain enclosure ratio [%] = (γ-phase length of crystal grain boundary / periphery length of crystal grain boundary) × 100
This “γ phase average crystal grain surrounding ratio” indicates the ratio of the γ phase distributed in a ring shape at the crystal grain boundary. Therefore, if the “γ phase average crystal grain surrounding ratio” is high, the probability that the crack contacts the γ phase is high. In addition, since the ratio of the γ phase distributed in a ring shape is shown, when the stress load direction is not specified, that is, when the crack direction is not specified, the γ phase necessary for suppressing the progress of the crack It is an appropriate index as a value indicating the distribution.

Next, the relationship between the “γ phase average crystal grain surrounding ratio” and the stress corrosion cracking resistance will be described based on actual measurement data.
A bar was manufactured from billets 1 to 3 having the same composition by three types of manufacturing methods, and a stress corrosion cracking test was performed on the bar. In addition, the γ-phase grain surround ratio, which is the ratio of the γ-phase surrounding the crystal grains, was analyzed from the microstructure and the correlation with the stress corrosion cracking resistance was obtained.
Table 3 shows the billet component values used in the test. The billet consisted of three different compositions for comparison. Moreover, the manufacturing method of the bar material manufactured from each billet in FIG. 5 is shown. In the figure, method A is a manufacturing method in which a billet is extruded and no heat treatment is performed, and method B is a manufacturing method in which a billet is extruded and then subjected to a pregelatinization heat treatment to provide dezincification corrosion resistance. Is a manufacturing method in which strain relief annealing is performed in order to improve elongation after extruding, and after passing through an alpha heat treatment, and method D is a manufacturing method in which annealing is performed after extrusion and drawing. The test material was a bar with a diameter of about 35 mm, and each annealing condition was 300 to 500 ° C. for about 2 to 4 hours.

Next, as shown in Table 4, the bars manufactured from billets 1 to 3 having different components manufactured by different methods A, B and C were used as test materials 1 to 6, and the γ phase average crystal grains in each test material The relationship between the surrounding ratio (%) and the stress corrosion cracking time (hr) measured by experiment is compared.
For the γ-phase grain-enclosed ratio, a microstructure photograph of 1000 times (length: 100 μm x width: 140 μm) was taken with an optical microscope, and the outer peripheral length of the crystal grains (the length of the grain boundaries) and the grain boundaries The length of γ phase existing in is measured and calculated by Equation 1.

FIG. 6 shows an example of the microstructure photograph at this time. FIG. 6A shows the description of the organization in the photograph. In FIG. 6B, the outer periphery of the crystal grain boundary at this time is indicated by a bold line, and in FIG. 6C, the length of the γ phase is indicated by a thick line. In FIG. 6B and FIG. 6C, the outer peripheral length of the crystal grain boundary (the length of the crystal grain boundary) and the length of the γ phase (the γ length of the crystal grain boundary) are measured. By applying 1 to the above, a crystal grain surrounding ratio of the γ phase that is a ratio of the γ phase to each crystal grain when the γ phase surrounds each crystal grain is calculated.
This was measured by arbitrarily selecting 20 crystal grains in one microstructure photograph, and the average value was taken as the average crystal grain surrounding ratio of the γ phase of the alloy. Table 4 shows the γ-phase average grain surround ratio and stress corrosion cracking time of each specimen obtained by this method. FIG. 7 is a graph showing the relationship between the γ-phase average crystal grain enclosure rate obtained from Table 4 and the stress corrosion cracking time.

From FIG. 7, the γ phase average crystal grain enclosure rate and the stress corrosion cracking time are substantially linear regardless of the difference in material and manufacturing method, and the stress corrosion cracking time increases as the crystal enclosure rate of the γ phase increases. It shows a tendency to become longer. In addition, from the relational expressions shown in the figure (y = 0.0.885x-10.695, R ² = 0.9632), the γ-phase average grain surround rate satisfying the standard B (stress corrosion cracking time 12 hr) is 28%. As described above, it was found that the γ-phase average grain surrounding ratio satisfying the more preferable criterion A (stress corrosion cracking time 26 hours) was 45% or more. Here, “R” in the above relational expression is a statistical “correlation coefficient”, and is expressed as an absolute value by using “R ² ” squared. The closer this R ² value is to 1, the closer the relational expression is to each data, that is, the stronger the correlation between x and y.
As shown in Table 3, this γ-phase average grain surround rate is adjusted by adjusting the alloy components (for example, adjusting the content of Cu and Bi) or by adjusting the presence or absence of annealing, annealing time, temperature, etc. It can be increased or decreased as appropriate, and can be set according to the standard of the target stress corrosion cracking time while maintaining the linear relationship with the stress corrosion cracking time shown in the above relational expression.

  As described above, by securing the average γ phase grain inclusion ratio of 28% or more, or 45% or more, the probability that the crack contacts the γ phase is increased. Shows the ratio of ring distribution in the crystal grain boundary, so when the stress loading direction is not specified, that is, in the alloy where the crack direction is not specified, the stress corrosion cracking resistance satisfying the predetermined standard Can be obtained. In addition, the upper limit of the γ phase average crystal grain surrounding ratio is about 75%, more preferably, the test material No. 3 in 71.

Here, although the number of measurements of the γ phase surrounding ratio necessary for calculating the γ phase average crystal grain surrounding ratio, that is, the number of crystals to be measured is an arbitrary numerical value, the number of measurements in this example was set to 20 This is because the average value calculated from the measured values is the minimum number of measurements necessary to converge to a certain value. As shown in FIG. 8, when the number of measurements is 1, the average value is the average value A, and when the number of measurements is 2, the average value B of the measured values a and b, the number of measurements 3 In this case, the average value C of the measured values a to c is obtained. In this example, based on the figure, the average value converges in the vicinity of 15 measurements, and taking the measurement error into consideration, the average value of the γ-phase crystal grain enclosure rate based on 20 measurements is expressed as γ Used as the phase average grain surround.
As described above, the necessary and minimum measured values can eliminate the influence of the variation in the average value, and can correctly grasp the correlation between the γ phase average crystal grain surrounding ratio and the stress corrosion cracking resistance.

Next, an example of the relationship between the number of γ-phase contacts and the stress corrosion cracking resistance will be described in detail.
“Number of γ-phase contacts” is the average value of data obtained by measuring the number of γ-phase contacts per unit length set in the direction perpendicular to the stress load direction at any part of the alloy and performing this measurement multiple times. Based on the standard deviation, it is defined by the following formula.
[Formula 1]
Number of γ-phase contacts [pieces] = “average value of the number of contacts in the γ phase” − “standard deviation of the number of contacts in the γ phase”
Therefore, if the “number of γ phase contacts” is high, the probability that the crack contacts the γ phase is high. In addition, this “number of γ-phase contacts” indicates the proportion of the γ-phase distributed in the direction perpendicular to the stress load direction. Therefore, when the stress load direction is specified, that is, the crack direction is specified. In this case, it is a suitable index as a value indicating the distribution of the γ phase necessary for suppressing the progress of cracking.
Here, the fact that the γ phase is distributed in the direction perpendicular to the stress loading direction is that stress corrosion cracking proceeds in the direction perpendicular to the stress loading direction.
As described above, Bi-based leadless copper alloys are prone to single and linear cracking, and therefore, in order to delay the progress of stress corrosion cracking, the γ phase is constant in the direction perpendicular to the stress load direction in the alloy. The stress corrosion cracking resistance is improved by efficient distribution according to the law.

Next, the relationship between the “number of γ phase contacts” and the stress corrosion cracking resistance will be described based on actually measured data.
In the same manner as in Example 1, bars were manufactured from billets 1 to 3 having the same structure by three types of manufacturing methods, and a stress corrosion cracking test was performed. In addition, the number of γ-phase contacts, which is the number of γ-phases present per unit length, was analyzed from the microstructure and the correlation with the resistance to stress corrosion cracking was determined.

  As shown in FIG. 9, the “number of contacted γ phases” is obtained by cutting a cylindrical specimen on a plane parallel to the stress load direction and using an optical microscope at a magnification of 400 times (any part of the cut surface). (Observation surface: 400 μm in length × 480 μm in width) The metal structure was photographed, and 24 straight lines having a length of 400 μm were drawn at 20 μm intervals in the direction perpendicular to the stress load direction on this photograph, and the γ phase contacting on the straight line The number of contacts was measured for each of the 24 lines, and the average value-standard deviation was calculated from the average value and standard deviation of the number of contacts to the γ phase in the 24 lines, and this was defined as “number of γ phase contacts”.

Here, the measurement was performed every 20 μm because the average crystal grain size of the test material was 14 to 16 μm, and multiple measurements on the same crystal grain were avoided. The unit length is set to 400 μm because the magnification at which the microstructure can be easily observed and measured is 400 times, and the short side of the visual field at this magnification is 400 μm.
Table 5 shows the number of γ-phase contacts (pieces) and the stress corrosion cracking time (hr) measured by experiments in each of the test materials 1 to 6. FIG. 10 is a graph showing the relationship between the number of γ-phase contacts obtained from Table 5 and the stress corrosion cracking.

From FIG. 10, the billet 2 and billet 3 had a linear relationship between the number of γ-phase contacts and the stress corrosion cracking time, and the stress corrosion cracking time tended to increase as the number of γ-phase contacts increased. Moreover, from the relational expression described in the figure, y = 5.9243x−2.637 and R ² = 0.9853, the number of γ-phase contacts satisfying the standard B (stress corrosion cracking time 12 hr) is 2 to 80 The number of γ-phase contacts satisfying the more preferable criterion A (stress corrosion cracking time 26 hr) is 4 to 80. Further, the billet 1 can also satisfy the criterion A by the number of 6 or more γ-phase contacts.

Here, the crystal grain size of the brass bar normally produced is about 5 μm when it is fine. As a result, a maximum of 80 crystals exist in the measurement length of 400 μm. Since one γ phase exists around one crystal grain, the upper limit value of the number of contacts with γ number is set to 80.
As shown in Table 4, this γ-phase contact number is adjusted by adjusting the alloy components (for example, adjusting the content of Cu, Bi, Sb), or by adjusting the presence or absence of annealing, annealing time, temperature, etc. It can be increased or decreased as appropriate, and can be set according to the standard of the target stress corrosion cracking time while maintaining the linear relationship with the stress corrosion cracking time shown in the above relational expression.

  In addition, in the “relationship between the γ phase average crystal grain sieving rate and the stress corrosion cracking time” in Example 1, it is not possible to grasp the influence of the inclusion of Sb on the stress corrosion cracking resistance from the graph of FIG. 10 is analyzed, the relationship between the Sb content and the stress corrosion cracking resistance can be quantitatively grasped.

  That is, in FIG. 10, the data of billet 2 (samples 3, 4, 5) and billet 3 (sample 6) containing Sb substantially conform to the equation y = 5.9243x−2.637. The data of billet 1 (samples 1 and 2) that do not contain Sb appears to deviate from the straight line, whereas in the case of the same number of γ-phase contacts, Sb is included. However, the stress corrosion time is improved compared to the case where it is not allowed. Thus, it has been found that it is better to contain Sb in that the stress corrosion cracking time becomes long even with a small number of γ-phase contacts.

  As described above, by securing the number of γ-phase contacts of 2 or more, or 4 or more (6 or more if Sb is not included), the probability of cracks contacting the γ-phase increases, and the γ-phase contact The number indicates the proportion of the γ-phase distributed in the direction perpendicular to the stress load direction. Therefore, when the stress load direction is specified, that is, in the alloy in which the crack direction is specified, Stress corrosion cracking resistance satisfying the standard can be obtained.

Here, “γ phase average crystal grain enclosure ratio” and “γ phase contact number” are values based on partial measurement data of the alloy, but as described later, the resistance of the alloy (test material) A clear correlation with the stress corrosion cracking property was obtained.
Based on this correlation, by appropriately setting the “γ phase average grain surround ratio” and “γ phase contact number”, the γ phase can be in a state of being distributed at a constant rate in the alloy, By making the probability of contacting the γ phase high with respect to cracking, the progressing speed of cracking can be slowed and the stress corrosion cracking resistance can be improved.
Then, just by calculating the “γ phase average crystal grain enclosure ratio” and “number of γ phase contacts” of the test material, the stress corrosion cracking resistance of the test material can be improved without performing a stress corrosion crack test each time. It is also possible to evaluate.

The “number of γ-phase contacts” is an index that can support the validity as a numerical value indicating that the probability that a crack is in contact with the γ-phase is high in terms of statistics.
As described above, the “number of γ-phase contacts” is an index calculated from the average value and standard deviation of the number of γ-phase contacts measured for a plurality of unit lengths.
In the index of only the average value, as shown in FIGS. 11 (a) and 12 (a), an alloy in which the γ phase is present on the average with respect to the unit length, or FIGS. 11 (b) and 12 (12). As shown in b), both of the alloys in which the γ phase varies with respect to the unit length show the same numerical value, and appropriately express the distribution of the γ phase necessary for suppressing the cracking speed. I can't.

  In addition, in the index of only the standard deviation indicating the variation in data, both the alloy having a large average value and the alloy having a small average value show the same numerical value as shown in FIGS. 13 (a) and 13 (b). After all, the distribution of the γ phase necessary for suppressing the cracking speed cannot be expressed appropriately.

  In the brass alloy according to the present invention, an index that combines the average value of the number of contacts of the γ phase and the standard deviation is used as an index that appropriately represents the existence state of the γ phase necessary for suppressing the cracking speed. As a result, the correlation with the stress corrosion cracking time can be found as described above, and the distribution of the γ phase necessary for securing the stress corrosion cracking of Bi-based leadless brass, which is an alloy exhibiting a linear crack, can be obtained. By being able to be identified, the validity as a numerical value indicating that the probability that the crack contacts the γ phase is high was confirmed.

  Further, since the “number of γ-phase contacts” is a numerical value represented by “average value (μ) −standard deviation (σ)”, the numerical value corresponding to the lower limit value of the shaded area in the normal distribution diagram of FIG. It is. In the normal distribution diagram of FIG. 14, the horizontal axis represents the number of γ-phase contacts, and the vertical axis represents the frequency at which each measurement data exhibits the number of γ-phase contacts.

  In statistics, as a means of inferring the data of the entire object (statistically referred to as “population”) based on partial measurement data of the object (statistically referred to as “sample”), A “normal distribution” that can commonly display the data distribution of many natural phenomena is used. In the alloy of the present invention, since it is necessary to estimate the distribution of the γ phase in the entire observation region based on the 24 measurement data in the observation region, the normal distribution diagram can be applied.

  According to this normal distribution, the probability that the number of γ-phase contacts with respect to the unit length, which is measurement data at an arbitrary position of the observation site, exhibits a numerical value equal to or greater than the “number of γ-phase contacts” is It shows that it is about 84%, corresponding to the shaded area.

  Therefore, in the brass alloy according to the present invention, “the number of contacts of two or more γ phases” means the number of contacts of γ phase when the number of contacts of γ phase per unit length is measured for 24 unit lengths. Means that there are 20 or more unit lengths.

  As described above, the “number of contact with γ phase” is an index that can support the validity as a numerical value indicating that the probability that the crack is in contact with the γ phase is high, and is as described above. Since a clear correlation with the stress corrosion cracking resistance of the whole alloy (test material) was obtained, as an index showing the distribution of the γ phase necessary for securing the stress corrosion cracking of Bi-based leadless brass, It is a valid numerical value.

Next, the relationship between the Sn content of the Bi-based leadless brass alloy in the present invention and the stress corrosion cracking resistance is investigated, and the optimum addition range (content) of the above Sn for the stress corrosion cracking resistance is proved. Therefore, the test of Example 3 was performed.
As a manufacturing method of the test materials 7-16 in this invention, the raw material was melt | dissolved in the high frequency furnace, and it poured into the metal mold | die at 1010 degreeC, and manufactured φ32 * 300 (mm) mold casting casting.

As a stress corrosion cracking test method, a stainless steel bushing in which a seal tape is wound around the Rc1 / 2 threaded portion of each specimen as shown in FIG. 2 is 9.8 N · m, as in the case of the evaluation standard test. The test was conducted by screwing in a desiccator containing ammonia water having an ammonia concentration of 14% at a test time of 4 to 48 hours. Subsequently, each specimen was taken out from the desiccator and washed every predetermined elapsed time (every 4, 8, 12, 24, 36, 48 hours), and then the presence or absence of cracks was evaluated by visual confirmation.
In Example 3, Table 6 shows the chemical components (mass%) of the manufactured castings (test materials 7 to 16) and the results of stress corrosion cracking time (hr) in each test material.

FIG. 15 is a graph showing the relationship between the Sn content and the stress corrosion cracking time in the test materials 7 to 14 (Bi is about 1.8%) obtained from Table 6.
From the result of FIG. 15, the tendency which cleared the evaluation criteria A (26h) determined in the above was shown in all the levels which added Sn more than 1.1 mass%. However, when Sn is added excessively, a nest is generated in the casting and workability is impaired. Therefore, the optimum addition range of Sn is preferably set to 1.0 to 2.0 mass%. On the other hand, as described above, the Sn content in the present invention is 0.7 to 2.5 mass%, but this content clears the standard B. In addition, the above-mentioned tendency is reproduced as shown in Table 6 also in the test materials 15 and 16 containing about 1.3 mass% of Bi.

Next, the relationship between the Sn content and the stress corrosion cracking resistance of the Bi-Se-based leadless brass alloy in the present invention was investigated.
Mold castings of the test materials No. 17 to No. 28 shown in Table 7 were manufactured, and a screw-in SCC test was performed. The test conditions are the same as those of the Bi-based brass test described above, and the screwing torque is 9.8 Nm, the ammonia concentration is 14%, and the time is 4 to 48 hours and n = 4. Moreover, in order to confirm also about the effect of Se, as shown to test material No.25 and No.26, it tested by Se: 0.09 % and 0.12%. The results are shown in Table 7 and the results of specimens Nos. 17 to 26 are also shown in FIG. In addition, in order to evaluate the test result for Bi-based brass and the test result for Bi-Se-based brass under the same conditions, reference test materials (Cu: 62.6, Sn: 0.3, The stress corrosion cracking time of Pb: 2.8, P: 0.1, Zn: balance, numerical unit is mass%) was evaluated. As a result, the stress corrosion cracking time of the standard specimen is 48 h in the test of Bi-based brass and 42 h in the test time of Bi-Se brass, so each specimen of Bi-Se brass Was multiplied by 48/42 = 1.14 as a correction value and displayed as “value after correction”.

  As a result of this test, it has been found that when Se is contained in addition to Sn, the stress corrosion cracking resistance is slightly improved. In addition, when the content of Se is increased as in test materials No. 20, No. 25, and No. 26, in the test material No. 26 (Se = 0.12%), stress corrosion resistance The cracking ability is slightly lowered and reaches a peak. In addition, the above-mentioned tendency is substantially reproduced as shown in Table 7 also in the test materials 27 and 28 containing about 1.3% of Bi.

  Next, the relationship between the Sb content of the Bi-based leadless brass alloy in the present invention and the stress corrosion cracking resistance is investigated, and the optimum addition range (content) of the above Sb with respect to the stress corrosion cracking resistance is proved. Therefore, the test of Example 5 was performed. The manufacturing method of the test materials 29 to 38 at this time is the same as that in Example 3.

As a stress corrosion cracking test method, a stainless steel bushing in which a seal tape is wound around the Rc1 / 2 threaded portion of each specimen as shown in FIG. 2 is 9.8 N · m, as in the case of the evaluation standard test. The sample screwed in at a torque of 1 is put into a desiccator containing ammonia water with an ammonia concentration of 14%, and each test material is taken out from the desiccator every time test time 4, 8, 12, 24, 36, 48 hours elapses. After cleaning, the presence or absence of cracks was evaluated by visual confirmation.
In Example 5, Table 8 shows the chemical composition (mass%) and the results of stress corrosion cracking time (hr) of the manufactured castings (test materials 29 to 38).

16 and 17 are graphs showing the relationship between the Sb content obtained from Table 8 and the stress corrosion cracking time. FIG. 16 is a bar graph showing the test results of each test material at equal intervals in order to show in detail the test results of the test material having a low Sb content, and FIG. 17 contains Sb. In order to show the general tendency of the specimens, the test results of the specimens are shown in a curve graph based on the Sb content.
From the results of FIGS. 16 and 17, by containing 0.06 to 0.60 mass% (more surely 0.06 to 0.51 mass%) of Sb, the stress corrosion cracking resistance satisfying the standard A is exhibited. To do. On the other hand, as described above, the Sb content in the present invention is 0.06 <Sb ≦ 0.60 mass%, but this content clears the standard B. In addition, in the sample material 30 (Sb: 0.02 mass%) and the sample material 31 (Sb: 0.04 mass%), the effect by containing Sb was not acquired.

  Here, the content of Sb in the alloy of the present invention is premised on the content of Sn: 0.7 to 2.5 mass%. As a comparative example with the above, Table 9 shows the results of a similar test performed on an alloy having a Sn content as low as 0.5 mass%. In these alloys, even when the Sb content was increased to 0.1 mass% and 0.3 mass%, no improvement in stress corrosion cracking resistance was confirmed.

なお、本発明におけるＢｉ−Ｓｅ系の鉛レス黄銅合金のＳｂ含有量と耐応力腐食割
れ性との関係について、Ｂｉ系の供試材と同様に試験を行った。

The relationship between the Sb content and the stress corrosion cracking resistance of the Bi-Se lead-free brass alloy in the present invention was tested in the same manner as the Bi-based test material.

表１０の結果より、Ｂｉ−Ｓｅ系の鉛レス黄銅合金においても、Ｂｉ系と同様の傾向が再現されている。

From the results in Table 10, the same tendency as in the Bi system is reproduced in the Bi-Se-based leadless brass alloy.

  Subsequently, in order to investigate the relationship between the Cu content of the Bi-based leadless brass alloy in the present invention and the stress corrosion cracking resistance and to determine the optimum addition range of Cu with respect to the stress corrosion cracking resistance, A test was conducted. The manufacturing method of the test materials 41 to 45 at this time is the same as in Example 3.

The test method for stress corrosion cracking is the same as in the case of Example 4. After removing the test material from the desiccator and cleaning it every test time 4, 8, 12, 24, 36, and 48 hours, visual confirmation is performed. The presence or absence of cracks was evaluated.
In Example 6, Table 11 shows the chemical components (mass%) of the manufactured castings (test materials 41 to 45) and the results of the stress corrosion cracking time (hr).

  FIG. 22 is a graph showing the relationship between the Cu content obtained from Table 11 and the stress corrosion cracking time. From the results of FIG. 22, it is effective to contain 59.5 mass% or more (more surely, 59.6 mass% or more) of Cu to satisfy the stress corrosion cracking resistance standard B (12h). In order to satisfy the standard A (26h), it was confirmed that inclusion of about 60.0 mass% or more (more certainly, 60.6 mass% or more) was effective.

  One of the causes of stress corrosion cracking is the residual tensile stress after processing the specimen. If this tensile stress remains, the stress corrosion cracking resistance may deteriorate due to the corrosive environment. Since Bi is an element that contributes to machinability, it affects the stress of the specimen remaining after processing. Therefore, the Bi content and the stress after processing of the specimen are investigated, and the Bi addition amount in which no tensile stress remains is determined. The manufacturing method of the test materials 46-50 at this time is the same as that of Example 3.

The stress of the test material is measured by the X-ray stress measurement method. Here, the stress from the outside affects the interval between the lattice planes constituting the material, and the grating distorted by the stress affects the angle of the diffracted X-ray with respect to the incident X-ray. The metal material is made of polycrystal, and when stress is applied to the metal material, the metal material generally expands in the direction of force and contracts in a perpendicular direction. Therefore, the internal stress can be obtained by measuring changes such as expansion and contraction of the distance between crystal lattice planes by the X-ray diffraction method.
In Example 7, the appearance and stress measurement location of the manufactured castings (up to specimens 46 to 50) are shown in FIG. 23, and the chemical component (mass%) and the measured stress value (MPa) are shown in Table 12. The shape of the casting is the same as that of the cylindrical specimen shown in FIG.

（＋は引張応力、−は圧縮応力を意味する）

(+ Means tensile stress,-means compressive stress)

  FIG. 24 is a graph showing the relationship between the Bi content obtained from Table 12 and the stress. From the results of FIG. 24, the stress tends to decrease as the Bi content increases. From the regression equation connecting the data with a straight line, it was found that when the Bi content was about 0.7 mass% or more, the residual stress after processing of the specimen changed to compressive stress.

  In addition, unless otherwise indicated, the stress corrosion cracking test in each Example is performed in the environment of about 20 degreeC.

Next, the distribution of Sb in the alloy will be described in detail.
As Example 5, the specimen 3 (α + β + γ structure) was subjected to mapping analysis by EPMA (electron beam microanalyzer), and the result is shown in FIG. At this time, the test material was manufactured by the method A in FIG. Mapping analysis was performed for six elements of Cu, Zn, Sn, Bi, Sb, and Ni in FIGS. 18A to 18F, respectively.

  When attention is paid to the Sb mapping image in FIG. 18E, white portions are observed in some places, and Sb is detected although the density is low. When this is compared with the other five elements, the white portion of Sb mostly corresponds to the black portion surrounded by white in the Sn mapping image in FIG. That is, Sb means that it exists in the same place as Sn.

Subsequently, quantitative analysis of the α phase, β phase, and γ phase in the alloy was performed by SEM-EDX (energy dispersive X-ray analysis method). The result is shown in FIG. FIG. 19 (b) shows the composition of the numerical analysis points in FIG. 19 (a).
Measurement locations (1) to (3) are the results of analyzing the γ phase. The γ phase is mainly composed of Cu, Zn, Sn, and Sb, Sn has a high concentration of about 10 mass%, and Sb is in solid solution at 3 mass%.

Next, the mapping analysis result by EPMA of the specimen 4 (α + γ structure) is shown in FIG. The sample material was manufactured by Method B in FIG. Mapping analysis was performed for six elements of Cu, Zn, Sn, Bi, Sb, and Ni in FIGS. 20 (a) to 20 (f), respectively.
When attention is paid to the Sb mapping image of FIG. 20E, (light) white portions are observed in some places, and Sb is detected although the density is low. When this is compared with the other five elements, the white portion of Sb corresponds to the black portion surrounded by the white color of the Sn mapping image in FIG. That is, it means that Sb exists in the same place as Sn, like the α + β + γ structure.

Subsequently, quantitative analysis of α phase, β phase, and γ phase in the alloy was performed by SEM-EDX. The result is shown in FIG. FIG. 21 (b) shows the composition of the numerical analysis points in FIG. 21 (a).
Measurement locations (3) to (6) are the results of analyzing the γ phase. The γ phase is mainly composed of Cu, Zn, Sn, and Sb, Sn has a high concentration of about 10 mass%, and Sb is in a solid solution of 2 to 3 mass%. Thus, the γ phase of the α + γ tissue was almost the same as the γ phase found in the α + β + γ tissue.
From the results of the above EPMA and SEM-EDX analysis, it can be said that Sb in the brass alloy having an α + β + γ structure and an α + γ structure is dissolved in the γ phase.

  Next, the billet 1 and billet 2 were manufactured by the method B, and the micro Vickers hardness of the γ phase found in the microstructure of the specimens 1 and 3 was measured at five locations. The γ phase was 158, the γ phase of the specimen 3 was 237, and it was revealed that the γ phase precipitated on the billet 2 was harder. As explained in the analysis results by EPMA or SEM-EDX, this is considered to be because the added Sb was dissolved in the γ phase. In this embodiment, the γ phase in which Sb is dissolved is distinguished from the γ phase of an alloy containing Sn in a brass alloy like Billet 1 and not containing Sb, and is defined as a “hardened γ phase”.

  The stress corrosion cracking property of a Bi-containing leadless brass alloy is important in how many γ phases are brought into contact with a linearly proceeding crack. Further, from the relationship between the number of γ phase contacts and the stress corrosion cracking time shown in FIG. 10, the bar containing Sb has a longer stress corrosion cracking time and less γ phase contact than the bar containing no Sb. Even the number results in a long stress corrosion cracking time. This means that the “cured γ phase” is more effective in preventing the progress of cracks than the “γ phase” against cracks that progress linearly.

Next, in order to evaluate the dezincification corrosion resistance and the erosion / corrosion resistance, the test materials 3 and 4 were subjected to a dezincification corrosion test and a crevice jet test.
(1) Dezincification corrosion test The dezincification corrosion test was carried out based on the dezincification corrosion test method for brass specified in ISO 6509-1981. Specifically, the surface is emery paper no. The test piece polished at 1500 was immersed in a test tank in which a 1% cupric chloride aqueous solution was maintained at 75 ° C. for 24 hours, and the corrosion depth and the corrosion form of the cross section of the test piece taken out from the test tank were measured with a microscope.・ Observed. The acceptance / rejection criteria were determined to pass the maximum dezincing depth of 200 μm or less (◎ in the table), pass over 200 μm and pass through to 400 μm (◯), and pass over 400 μm as reject (x).
As shown in Table 13, all the test materials passed.

(2) Crevice jet test The erosion / corrosion resistance was evaluated by a crevice jet test. Specifically, a test piece processed to have an exposed area of 64πmm ² (φ16 mm) with respect to the corrosive solution is mirror-polished and arranged as shown in FIG. Subsequently, a test solution (1% cupric chloride aqueous solution) is injected at 0.4 liter / min from an injection nozzle (nozzle diameter: φ1.6 mm) disposed at a height of 0.4 mm from the surface of the test piece. After spraying the test solution for 5 hours, the mass is measured to determine the mass loss and the corrosion depth, and the corrosion form is observed. The pass / fail criterion is that the test material that does not show local corrosion compared to the comparative bronze casting is acceptable (O in the table), and the test material that shows local corrosion is rejected. .
As shown in Table 14, all the test materials passed.

  As described above, the brass alloy of the first invention contains Sb as in billet 2 in Table 3, and this is subjected to heat treatment for pre-annealing by method B in FIG. I was able to improve. At this time, excellent dezincification corrosion resistance and erosion / corrosion resistance, which are the characteristics of brass alloys, could be secured.

  Next, the preferred embodiment in the lead-less brass alloy excellent in stress corrosion cracking resistance in the second invention will be described in detail. The lead-less brass alloy of the second invention precipitates a γ phase by containing Sn in the Bi-based lead-less brass alloy, and this γ phase is uniformly dispersed in the metal structure. It is a lead-less brass alloy that is improved in stress corrosion cracking resistance by suppressing local corrosion on the alloy surface by becoming a corroding site.

  The elements contained in the lead-less brass alloy in the second invention and the desirable composition range thereof are the same for the composition range and the reason in the first invention, and thus the description thereof is omitted. In order to uniformly disperse the γ phase, by using an appropriate preferable manufacturing method among the manufacturing methods A to D shown in FIG. 5, α + γ shown by cross-hatching in the state diagram of FIG. A tissue (see range S) or α + β + γ tissue (see range R) indicated by hatching is obtained. In particular, as in methods B to D, by carrying out pre-annealing annealing and suppressing the β phase, the γ phase is uniformly dispersed and the stress corrosion cracking resistance is improved while having dezincing resistance. Is possible.

Here, an evaluation method using an “evaluation coefficient” will be described as means for selecting an appropriate preferable manufacturing method required for uniform dispersion of the γ phase in the lead-less brass alloy in the second invention.
“Evaluation coefficient” is a numerical method using statistical means to evaluate the effect of manufacturing processes (factors) such as drawing and heat treatment on stress corrosion cracking resistance in a method of manufacturing a lead-free brass alloy bar ( Weighted), which is the value expressed as the product of these numerical values.
For example, a rod with a diameter of φ32 manufactured through the steps of “extrusion” and “pre-annealing annealing (temperature: 470 ° C.)” is used, and without performing “drawing” from this rod, “heat treatment both before and after drawing” As an example of calculating the evaluation coefficient of a test sample manufactured without performing 1 as a reference value, the evaluation coefficient can be expressed by the following equation.
[Formula 2]
“Evaluation coefficient” = effect of bar diameter × effect of α-izing temperature × effect of drawing × effect of performing heat treatment both before and after drawing = a / 32 (1+ | 470−t | / 100) × (drawing Perform: 0.8) x (heat treatment is performed both before and after drawing: 0.3)
Here, a is a rod diameter (unit: mm), t is a pregelatinization temperature (° C.), and the evaluation coefficient is a dimensionless number. Further, when the pre-annealing is not performed, the influence of the pre-heating temperature (1+ | 470−t | / 100) is set to 1.

Using billets having the chemical components shown in Table 15, through the production steps shown in Table 16 (annealing before drawing, drawing, annealing after drawing), the first to fifth specimens 51 to 73 of each bar diameter were manufactured. The stress corrosion cracking test similar to Example 3 in the invention was performed, and the evaluation coefficient was calculated using Equation 2. The stress corrosion cracking time (SCC time), which is the result of the stress corrosion cracking test, and the calculated evaluation coefficient are shown in Table 16, and the relationship between the stress corrosion cracking time and the evaluation coefficient is shown in the graph of FIG.

As shown in FIG. 27, the evaluation coefficient and the stress corrosion cracking time are in a substantially linear relationship that rises to the right, indicating that the SCC time tends to increase as the evaluation coefficient increases. In addition, the relational expression (y = 39.657x × −6.2186, R ² = 0.9113) shown in the figure indicates that the correlation between the evaluation coefficient and the SCC time is high. According to FIG. 27, the evaluation coefficient satisfying the criterion B (stress corrosion cracking time 12 hr) is 0.46 or more, and the evaluation coefficient satisfying the criterion A (stress corrosion cracking time 26 hr) is 0.81 or more.

In FIG. 60, no. 69, no. 70 shows microstructure photographs (observed at 200 and 1000 times). The evaluation coefficient and the stress corrosion cracking time in each specimen are 1.50-46 hr, 0.78-26 hr, 0.23-3.3 hr, and these are (a) and (b) in the graph of FIG. ), (C) corresponds to the area.
The observation site of the microstructure is a longitudinal cross-sectional structure after the stress corrosion cracking test in the vicinity of the Rc1 / 2 female thread portion of the test material of the stress corrosion cracking test shown in FIG. This structure shows the microstructure in the extrusion longitudinal direction of the bar, and the stress corrosion cracking time becomes shorter as the γ phase existing so as to surround the crystal grains is distributed in the state aligned in the photo longitudinal direction. Is shown.

Specimen No. In No. 60, the α-treatment is performed at 425 ° C., which deviates from the optimum temperature described later, and since the β phase remains, the distribution of the γ phase is good, the stress corrosion cracking time is long, and the stress resistance Excellent corrosion cracking property.
Specimen No. In No. 69, the α-treatment is performed at 450 ° C., which is close to the optimum temperature, and there is almost no β phase remaining, so although the γ phase tends to be aligned in the longitudinal direction, the stress corrosion cracking resistance is observed. The property is good.
Specimen No. No. 70 is heat-treated both before and after drawing, and the tendency of the γ phase to align in the longitudinal direction further advances, and the stress corrosion cracking time is also short.

Next, each factor of the evaluation coefficient will be described.
(1) Influence of bar diameter (reference value in formula 2: φ32)
“Influence of bar diameter” is a factor that increases or decreases the relative value of the evaluation coefficient, and does not directly affect the relationship between the evaluation coefficient and the stress corrosion cracking time. For example, the graph of FIG. 29 shows the relationship between the evaluation coefficient and the stress corrosion cracking time when the reference value of the bar diameter is φ1, that is, when the influence of the bar diameter is a / 1. Then, when the reference value is φ1, the evaluation coefficient value is larger than that of the graph 30 when the reference value is φ32, and the slope and intercept of the graph change, but the evaluation coefficient and The value of “correlation coefficient R ² ” indicating the correlation with the stress corrosion cracking time is not changed.
Therefore, “the influence of the bar diameter” does not directly affect the relationship between the evaluation coefficient and the stress corrosion cracking time, and is a numerical value that can be appropriately selected by the evaluator according to the purpose. It is an arbitrary factor.

(2) Effect of pregelatinization temperature (reference value in Formula 2: 470 ° C.)
“Effect of pregelatinization temperature” is a factor that increases or decreases the substantial value of the evaluation coefficient, and slightly affects the relationship between the evaluation coefficient and the stress corrosion cracking time. In the lead-less brass alloy of the present invention, the dezincing resistance is improved at 455 ° C. <t <475 ° C. (more surely 485 ° C.), which is optimum for the pregelatinization temperature. There exists a tendency for SCC property to fall.
As a specific example, a billet having chemical component values shown in Table 15 was used, and after a specimen having a rod diameter of φ33 was produced by extrusion, a stress corrosion cracking test similar to Example 3 in the first invention was performed. FIG. 30 is a graph showing the relationship between the pregelatinization temperature and the stress corrosion cracking time. Although there is some variation in each data, the shortest stress corrosion cracking time (SCC time) is the data at 470 ° C. Therefore, as an appropriate preferable manufacturing method required for uniform dispersion of the γ phase, an α-treatment By carrying out at a temperature higher or lower than 470 ° C., it is possible to suppress a decrease in stress corrosion cracking resistance. In view of the balance between the stress corrosion cracking resistance and the dezincing resistance, it is optimal to perform the alpha treatment at 425 ° C to 455 ° C.
Therefore, “the influence of the pregelatinization temperature” slightly affects the relationship between the evaluation coefficient and the stress corrosion cracking time, and is an arbitrary factor in the “evaluation coefficient”.

(3) Influence of drawing (degree of influence: 0.8)
“Influence of drawing” is a factor that increases or decreases the substantial value of the evaluation coefficient, and affects the relationship between the evaluation coefficient and the stress corrosion cracking time. In general, it is said that the stress corrosion cracking resistance of brass alloys is improved by increasing the tensile strength and proof strength by drawing, but the toughness such as elongation and impact tends to decrease. However, when a notch is generated on the surface due to corrosion, there is a possibility that the cracks may advance quickly.
FIG. 31 shows another example in which the degree of influence of drawing is 0.6. In this graph, since the correlation coefficient is R ² = 0.8942, the correlation between the evaluation coefficient and the SCC time is slightly lower than in the case of FIG. 27 in which the degree of influence of drawing is 0.8. It will be a thing. In order to set the correlation coefficient to 0.9 or more, it is preferable to set the degree of influence of drawing between 0.6 and 0.9 (example: phase when the degree of influence of drawing is 0.9) The number of relationships was R ² = 0.8997).
As an appropriate and preferable manufacturing method required for uniform dispersion of the γ phase, the stress corrosion cracking resistance can be improved by proceeding to the next step such as the α-treatment without drawing.
Therefore, “the effect of drawing” affects the relationship between the evaluation coefficient and the stress corrosion cracking time, and is an essential factor in the “evaluation coefficient”.

(4) Influence of heat treatment before and after drawing (degree of influence: 0.3)
“Effect of performing heat treatment both before and after drawing” is a factor that increases or decreases the substantial value of the evaluation coefficient, and greatly affects the relationship between the evaluation coefficient and the stress corrosion cracking time.
32 and 33 are graphs showing changes due to the degree of influence of performing heat treatment both before and after drawing. The degree of influence in FIG. 32 is 0.4 or less, the best is 0.3, and FIG. 0.3 and FIG. 33 is 0.2. As is clear from the figure, the correlation coefficient is increased by reducing the degree of influence. Table 17 below shows combinations of upper and lower limits of evaluation coefficient factors and evaluation coefficient boundary values.

  Table 17 shows the upper and lower limit values of each factor affecting the stress corrosion cracking resistance and evaluation coefficients corresponding to the criteria A and B. By changing each factor of the evaluation coefficient from the table, the evaluation coefficient for the reference A can be 0.70 to 0.89, and the evaluation coefficient for the reference value B can be 0.29 to 0.58. This indicates that it varies depending on differences in manufacturing equipment and manufacturing conditions for bar materials, as well as variations in the results of stress corrosion cracking tests, but generally the distribution of γ phase is good by setting the optimum values for each factor. Thus, an alloy having excellent stress corrosion cracking resistance is obtained, and the evaluation coefficient corresponding to the reference A at this time is 0.81 and the reference B is 0.46.

32, 33, and Table 17, the phase transformation easily proceeds when heat treatment is performed in a state where the residual stress of the material is high, and in the case of the brass alloy in the present invention, extrusion → pre-annealing → drawing → strain relief annealing, etc. By repeating the high strain processing and the heat treatment twice, the distribution of the γ phase is deteriorated and the possibility that the SCC resistance is lowered becomes extremely high.
The influence of performing heat treatment both before and after drawing can be set from the correlation coefficient of the regression line of the graph showing the evaluation coefficient and the stress corrosion cracking time. Based on the setting in a range where a high correlation is obtained (correlation coefficient R2 is 0.9 or more), it is preferable to set the influence of performing heat treatment both before and after drawing to 0.4 or less (see FIG. 32). ). A high correlation coefficient can be obtained by bringing the effect of heat treatment before and after drawing closer to 0. The evaluation coefficients of 54, 55, 56, 57, 67, 70, 72 and 73 are close to 0, indicating that the stress corrosion cracking time is about 0.0 hr. Here, in Table 14, No. 54 and 57 indicate that the stress corrosion cracking time is 0.0 hr, but in actuality, it means that all the test pieces were cracked in less than 4 hr. In other words, since the stress corrosion cracking time becomes 0.0 hr contradictory, it is not preferable to make the influence of performing the heat treatment both before and after drawing near zero. From the above, it is preferable that the lower limit of the effect of performing heat treatment both before and after drawing is 0.2 (see FIG. 33). Further, it is best to set the influence of performing heat treatment both before and after drawing to 0.3 (see FIG. 27).

Further, as an appropriate preferable manufacturing method required for uniform dispersion of the γ phase, as shown in manufacturing methods B and D in FIG. 5, when heat treatment is performed, the heat treatment is performed only before or after the drawing. Thus, the resistance to stress corrosion cracking can be improved.
Therefore, “the effect of performing heat treatment both before and after drawing” has a great influence on the relationship between the evaluation coefficient and the stress corrosion cracking time, and is an essential factor in the “evaluation coefficient”.
As described above, by evaluating using the “evaluation coefficient”, it becomes easy to select a preferable manufacturing method required for uniform dispersion of the γ phase in the lead-less brass alloy in the second invention, and a desired resistance resistance. A lead-less brass alloy having stress corrosion cracking properties can be obtained efficiently.

Next, the corrosion in the second invention will be described. Corrosion in the second invention means that a metal reacts with water or oxygen in the environment to rust, discolor the surface and wear, and is classified into full-surface (uniform) corrosion and local corrosion.
As shown in FIG. 34 (a), the general corrosion is that the wear (corrosion) of the metal surface proceeds uniformly, and during this total corrosion, both the anode reaction and the cathode reaction proceed uniformly on the metal surface. It is supposed to be.

  On the other hand, local corrosion is a corrosion form in which one component in the alloy component is selectively dissolved as shown in FIG. 34 (b), and this form occurs when the anodic reaction occurs in a concentrated portion on the metal surface. It becomes. At this time, the cathode part is in a passive state in which metal dissolution hardly proceeds, and only the oxygen cathode reduction reaction proceeds at this part. On the other hand, the anode part is in an active state where metal dissolution easily occurs, and only the anode reaction proceeds at this part. At this time, since the area of the anode part is usually extremely small compared to the area of the cathode part, the corrosion current density at the anode part becomes extremely large, thereby causing severe local corrosion.

At this time, in the state of local corrosion, the stress tends to concentrate on the significantly corroded portion, and the time until the crack occurs is shortened. On the other hand, in the case of full-surface corrosion, the alloy surface is uniformly corroded to relieve stress concentration, so that the time until crack generation is longer than that of local corrosion.
That is, in order to alleviate the stress concentration, it is important to take the form of general corrosion, and for that purpose, it is important to control the distribution, abundance, shape, etc. of the intervening phase that can become the anode part. As parameters for controlling these, (1) the dispersion degree of the intervening phase, (2) the circularity of the intervening phase, and (3) the α phase aspect ratio were used. Each parameter will be described below.
Here, the intervening phase refers to a component or an intermetallic compound that does not dissolve in the α phase or β phase, and examples thereof include a Bi phase, a Pb phase, a γ phase, and a ZnSe phase. In particular, in the description of the parameters shown below, it refers to a γ phase or Pb phase that is preferentially corroded compared to the α phase.

  Since stress corrosion cracking is a phenomenon that occurs when the corrosion depth reaches a specific depth (see dimension L in FIG. 34 (b)), this corrosion gradually and uniformly progresses over the entire metal surface. By taking the form of so-called overall corrosion, the time until the corrosion reaches a specific depth can be delayed, and the occurrence of cracks can be suppressed. As an example of this specific depth, the maximum corrosion depth (for example, maximum corrosion depth at a corrosion time of 144 h = about 59.4 μm) of the present invention product in Table 24 of Example 17 described later corresponds.

(1) Dispersion degree of intervening phase In order to determine the dispersion degree of the intervening phase, in this example, as a predetermined range, 19 × 19 squares (one square is 13 μm × 17 μm) are formed on a 400-times microstructure photograph. The value of (number of cells in which intervening phases exist) / (total number of cells 361) was measured, and this average value was calculated with n = 5. This calculation result is used as the degree of dispersion of the intervening phase. The degree of dispersion of the intervening phase is an index for expressing how much the intervening phase is dispersed, and the closer to 1, the better the dispersion. I mean. Further, since the degree of dispersion is low when the amount of the intervening phase is small, the element of the amount of the intervening phase is included.

(2) Circularity of the intervening phase The circularity of the intervening phase was measured by the graphite shape factor method using the measurement principle of the spheroidization ratio of graphite in spheroidal graphite cast iron. In this example, n = 30 was measured and the average value was calculated. The circularity of the intervening phase is an index representing the shape of the intervening phase, and it means that the closer to 1, the more true the circle is, and the farther away, the farther from the true circle. In addition, since the amount of the intervening phase is very small, it becomes close to a perfect circle, and therefore includes an element of the amount of the intervening phase.

(3) α phase aspect ratio The α phase aspect ratio was obtained by measuring the aspect ratio of the α phase on the alloy surface. In this example, n = 30 was measured and the average value was calculated. As shown in FIG. 35, when the vertical length of the α phase is a and the horizontal length is b, when the α phase aspect ratio a: b is close to 1, the α phase is as shown in FIG. It becomes a perfect circle and becomes vertically long as shown in FIG. Furthermore, when the α phase aspect ratio is close to 1, the intervening phases are distributed so as to surround the α phase grain boundaries. On the other hand, when the aspect ratio is large, the γ phase tends to exist side by side in the vertical direction. That is, the α phase aspect ratio includes elements of the degree of dispersion and shape of the intervening phase.

  Subsequently, the relationship between the three parameters of the dispersity of the intervening phase, the circularity of the intervening phase, and the α phase aspect ratio and the stress corrosion cracking resistance is derived. In order to derive the relationship between the parameters and the stress corrosion cracking resistance, each parameter of the brass alloy of the second invention is measured. Moreover, in order to compare with the brass alloy of this invention, it measures similarly about the brass alloy which consists of another chemical component value.

  The brass alloy in the second invention is a brass alloy (hereinafter referred to as “the product of the present invention”) provided according to the chemical component values shown in Table 18 as an example. Further, brass alloys (hereinafter referred to as “comparative examples”) 1, 3, and 4 for comparison were similarly provided according to chemical component values in Table 18.

  For the product of the present invention (second invention) and a comparative example, the dispersity of the intervening phase, the circularity of the intervening phase, and the α-phase aspect ratio were measured using a sample having a material diameter of φ32. In a desiccator in a% ammonia atmosphere, the time to break when a tensile force of 50 MPa was applied to each sample was examined, and the results are shown in Table 19. The test method of this tensile SCC property test is the same as the examples described later.

Here, the intervening phases to be measured in each sample are the γ phase in the product of the present invention and Comparative Example 3, the Pb phase in Comparative Example 1, and the γ phase and the Pb phase in Comparative Example 4.
In addition, the “tensile direction” and “observation surface” in Table 19 refer to the direction in which a tensile force is applied to the specimen extracted from the bar and the parameter measurement surface, as shown in FIG. In this example, the product of the present invention was manufactured by manufacturing method A in Table 5. Hereinafter, Comparative Example 1 is Manufacturing Method B, and Comparative Example 2 (see Table 20) is Manufacturing Method A. Example 3 is manufactured by Manufacturing Method C, and Comparative Example 4 is manufactured by Manufacturing Method A.

※ｘ：分散度／（介在相円形度×アスペクト比）

* X: Dispersion / (Intervening phase circularity x aspect ratio)

  Subsequently, x in Table 19 (dispersity of intervening phase / (circularity of intervening phase × α phase aspect ratio)) is taken as the X axis, and the fracture time in the tensile SCC property test is taken as the Y axis, and the measurement results of each sample are plotted. did. The result is shown in FIG. 37 as the relationship between the structure parameter and the tensile SCC test result (breaking time).

As shown in FIG. 37, the products 15 and 16 of the present invention have other values when x (dispersity of the intervening phase / (circularity of the intervening phase × α phase aspect ratio)) is 0.5 or more with respect to the comparative example 13. From the regression line L of the plotted measurement results, it can be judged that the stress corrosion cracking resistance (breaking time) is superior to that of the comparative example, that is, X ≧ 0.5 and Y ≧ 135.8X-19. It was confirmed that the alloy satisfying the relational expression can exhibit the stress corrosion cracking resistance equal to or higher than that of Comparative Example 13. Further preferably, the value of 1.09 or more which is the value of x of the product 15 of the present invention, that is, Brass alloy having a structural parameter of dispersion degree of intervening phase satisfying the relational expression of X ≧ 1.09 / (circularity of intervening phase × α phase aspect ratio) (brass alloy in a region represented by hatching in FIG. 37) ) Is more desirable.
In addition, although the comparative example 14 is also plotted in the position which satisfy | fills said relational expression in this figure, since this comparative example 14 (comparative example 13) is the comparative example 1 of Table 18, content of Sn is low, It deviates from the premise of high Sn content, which is the premise of the present invention.

As described above, it has been found that there is a high correlation between the degree of dispersion of the intervening phase / (α phase aspect ratio × the degree of circularity of the intervening phase) and the tensile SCC rupture time, and a parameter indicating the uniform dispersion of the γ phase. I was able to find this relationship. By setting this parameter to an appropriate value, the anode part and the cathode part in the alloy can be distributed in a balanced manner, and the γ phase can be uniformly dispersed.
As a result, the lead-less brass alloy of the present invention uniformly disperses the γ phase in the alloy structure, and the anode-cathode reaction is substantially reduced on the alloy surface by the γ phase that reacts as the anode site and the α phase that reacts as the cathode site. It is trying to progress uniformly.

"Evaluation by maximum corrosion depth / average corrosion depth"
Next, the stress corrosion cracking resistance of the brass alloy in the present invention is analyzed from the point of the state of corrosion. As an example, a brass alloy having a chemical component value as shown in Table 20 was provided, and the maximum corrosion depth and the average corrosion depth after corrosion of this product and Comparative Examples 1, 2, and 4 were measured in Example 11 to be described later. These were expressed numerically by the ratio of maximum corrosion depth / average corrosion depth, and the local corrosion inhibition state was expressed. Table 21 and FIG. 38 show the ratio of the maximum corrosion depth and the average corrosion depth of the products of the present invention in Table 20 and Comparative Examples 1, 2, and 4. Here, the crystal structure of the product of the present invention is (α + β + γ) + Bi, Comparative Example 1 is lead-containing dezincing-resistant brass, the crystal structure of which is (α) + Pb, and Comparative Example 2 is lead-containing free-cutting brass. The crystal structure was (α + β) + Pb, Comparative Example 4 was lead-containing dezincing-resistant brass, and the crystal structure was (α + β + γ) + Pb.

  In Table 21, the ratio of the maximum corrosion depth / average corrosion depth indicates that the closer to 1, the more general corrosion is exhibited. In the product of the present invention, this ratio is a small value, and variations due to the passage of corrosion time are reduced. On the other hand, in Comparative Examples 1, 2, and 4, this ratio is a relatively large value, and variation due to the passage of the corrosion time is large. From these tendencies, the product of the present invention exhibits general corrosion, indicating that there is no change in the corrosion form with the passage of the corrosion time.

  Further, when a tensile SCC property test similar to that of Example 12 described later was performed in a 14% ammonia atmosphere and a load stress of 50 MPa, as shown in Table 21, the product of the present invention: 157.3h, Comparative Example 1: 41. 7 h, Comparative Example 2: 21.3 h, Comparative Example 4: 33.2 h. From this result, it is considered that in the comparative example, the initial corrosion state up to about 24 hours of corrosion time is related to the fracture time. Thereby, when the value of the maximum corrosion depth / average corrosion depth up to 24 hours of corrosion is compared, the products of the present invention are 3.8 to 4.2, whereas Comparative Examples 1, 2, and 4 are Both are above this value. Among these, if Comparative Example 1 with the longest rupture time is set as a comparison object, the Comparative Example 1 has a maximum corrosion depth / average corrosion depth ratio of 1 to 8.6 at a corrosion time of 24 h. This corrosion at the initial stage tends to be a starting point of cracking. Moreover, since corrosion will increase on average after a long period of time, it is difficult to judge. Therefore, each sample material can be correctly evaluated by comparison in the initial stage up to 24 h.

Therefore, the brass alloy of the present invention has a 14% ammonia atmosphere and a load stress as long as the maximum corrosion depth / average corrosion depth is in the range of 1 to 8.6 within a corrosion time of 24 hours. Under the condition of 50 MPa, stress corrosion cracking resistance equal to or higher than that of the comparative example can be provided.
Furthermore, more preferably, the overall corrosion state having a maximum corrosion depth / average corrosion depth of 1 to 3.8 which is the range of the test result of 24 h of the present invention product is good. In addition, when the time until fracture is an object of evaluation, the maximum corrosion depth / average corrosion depth from 1 to the maximum value of 6.4 is preferably included from the results in Table 21.

  In addition, when the fluctuation rate of the maximum corrosion depth / average corrosion depth (maximum value / minimum value) × 100 within the corrosion time 144 h is calculated, the product of the present invention is 110% in Table 21, while Comparative Example 1 is About 163%, Comparative Example 2 is 166%, and Comparative Example 4 is about 212%. The product of the present invention is smaller than the Comparative Example. Moreover, the maximum corrosion depth / average corrosion depth value in the initial corrosion state up to 24 h is the smallest among the four specimens. Accordingly, the product of the present invention is in a state of overall corrosion with a fluctuation rate of 110% or less, and continues to be in a state where the maximum corrosion depth is small even with time, and local corrosion is suppressed.

"Evaluation by coefficient of variation"
Subsequently, when it is considered that the entire surface corrosion form occurs when the variation in the corrosion depth is small, the standard deviation indicating the variation in the data with respect to the corrosion depth and the average value of the product of the present invention and each comparative example is obtained, and evaluation is performed by the coefficient of variation. This is analyzed. However, since the standard deviations of different groups cannot be simply compared, the variation in corrosion depth was compared using a coefficient of variation. The coefficient of variation is a value obtained by dividing the standard deviation of the corrosion depth within a predetermined range by the value of the average corrosion depth within the range, and can provide a standard for the corrosion depth when the alloys are compared. Therefore, by comparing the coefficient of variation, the products of the present invention, which are different groups, and the corrosion depth variation of each comparative example were compared.

  For the product of the present invention and Comparative Examples 1, 2, and 4, the standard deviation when the corrosion depth is measured at n = 30 is divided by the value of the average corrosion depth, and the obtained variation coefficient is shown in Table 22 and FIG.

  In Table 22 and FIG. 39, as in the case of comparing the maximum corrosion depth / average corrosion depth, the value of the coefficient of variation until the corrosion time is 24 h is the product of the present invention: 0.77 to 0.79. There is a small variation in the coefficient of variation in this way, so that the variation in the corrosion depth is small, and the corrosion progresses uniformly.

  On the other hand, in the comparative example, the coefficient of variation becomes Comparative Example 1: 1.70 to 1.81, Comparative Example 2: 1.18 to 1.39, Comparative Example 4: 1.25 to 1.39. The variation is larger than that of the invention, and it can be determined that this is a form of local corrosion. In the same manner as described above, when Comparative Example 2 is set as a comparison target, the variation coefficient of Comparative Example 2 is 1.18 at a corrosion time of 24 h. Therefore, the brass alloy of the present invention has a 14% ammonia atmosphere and a load stress of 50 MPa under the condition that the coefficient of variation takes a value greater than 0 and 1.18 or less over a corrosion time of 24 hours. It can have the stress corrosion cracking resistance equivalent to or better than that of the comparative example.

More preferably, the coefficient of variation should be 0.77 or less, which is the range of the 24 h test result of the product of the present invention. In addition, when the time until fracture is an object of evaluation, the maximum value of the coefficient of variation is preferably set to 0.62 based on the results in Table 22.
As described above, the corrosion state can be quantified by the maximum corrosion depth / average corrosion depth and the coefficient of variation, and the corrosion state can be quantified and compared by these different comparison means.

  Next, each Example is demonstrated according to drawing about the evaluation test of the corrosion form of the brass alloy excellent in the stress corrosion cracking resistance in the second invention, and the stress corrosion cracking test.

First, the difference in corrosion form between the brass alloy of the present invention and the conventional brass alloy under stress corrosion will be verified. In order to investigate the difference in the corrosion form of each brass material in a stress corrosion cracking environment, the product of the present invention in Table 20 and Comparative Examples 1, 2, and 4 were placed in a desiccator in a 14% ammonia atmosphere for 24 hours, and thereafter The microstructure of the cross section was observed at a magnification of 200 times. A microstructure cross section before and after the corrosion test is shown in FIG.
As a result, it was confirmed that the corrosion form of the product of the present invention was uniform corrosion because local corrosion was suppressed and the entire surface layer was corroded. On the other hand, Comparative Examples 1 and 2 can be judged as local corrosion because they are locally corroded. Moreover, although the comparative example 4 is corroded uniformly, there exists partial deep corrosion and it is in the state close | similar to local corrosion.

  From Example 10, the difference in the corrosion form due to the difference in the chemical component value was confirmed. Next, in order to identify the intervening phase that is preferentially corroded in the stress corrosion cracking environment, the structure form of (α + β + γ) is used. Corrosion tests were performed on certain Bi-containing brass (product of the present invention) and Pb-containing brass (Comparative Example 4).

  In the test, the product of the present invention and Comparative Example 4 were placed in a 14% ammonia atmosphere for 24 hours, and the surfaces before and after corrosion were observed. At this time, in order to specify the intervening phase to be corroded, an indentation was made by a micro Vickers tester so that the same portion could be observed before and after the corrosion. A photograph before corrosion taken at a magnification of 1000 is shown in FIG. 41, and a photograph after corrosion is shown in FIG. As a result, it was observed that the γ phase was corroded for the product of the present invention, and the γ phase and Pb were corroded for Comparative Example 4. On the other hand, the β phase and Bi phase were not corroded. Thereby, it was confirmed that the intervening phases corroded preferentially compared with the α phase are the γ phase and the Pb phase. In particular, it was confirmed that the γ phase is preferentially corroded even when compared with the Pb phase.

Further, for the product of the present invention and Comparative Examples 1, 2, and 4, a cross section of the microstructure before and after corrosion was photographed at a magnification of 400 times. The result is shown in FIG. In the structure before corrosion of the product of the present invention, the γ phase is uniformly distributed on the surface layer. On the other hand, in Comparative Examples 1 and 2, Pb is distributed in the vicinity of the surface layer, and in Comparative Example 4, the γ phase and Pb are distributed. Further, in the product of the present invention after corrosion, the γ phase near the surface layer is uniformly corroded. On the other hand, in Comparative Examples 1 and 2, Pb near the surface layer is locally corroded, and Comparative Example 4 is uniform corrosion, but since the γ phase and Pb are corroded, the corrosion depth becomes deeper. Yes.
Thus, it was demonstrated that uniform dispersion of the γ phase without containing Pb is a solution for preventing local corrosion of the brass alloy and causing uniform corrosion.

  In order to verify the relationship between the corrosion time and the corrosion depth in the stress corrosion cracking environment for the product of the present invention and Comparative Examples 1, 2, and 4, a corrosion test was performed to confirm the presence or absence of local corrosion. In the test, each test piece was placed in a 14% ammonia atmosphere, taken out after 24 hours, 86 hours, and 144 hours after the start of the test, and the corrosion depth was measured. The corrosion depth was measured using a dezincification corrosion depth measurement method. In this measurement method, the average corrosion depth was determined by measuring 6 points of the microstructure of the sample after the corrosion test (n = 3) at a magnification of 200 times, measuring the 5-point corrosion depth at equal intervals per location, and 30 The average value of the points was determined. The maximum corrosion depth was measured at the point where the corrosion depth of the photographed microstructure image was maximum.

  The relationship between the corrosion time and average corrosion depth of each alloy is shown in Table 23 and FIG. 44, and the relationship between the corrosion time and maximum corrosion depth is shown in Table 24 and FIG. In any alloy, the average corrosion depth gradually increases with time, and in particular, the corrosion depth of Comparative Example 4 increases. Moreover, although the maximum corrosion depth of Comparative Examples 1, 2, and 4 becomes large with progress of time, the maximum corrosion depth of this-invention product is changing with the constant corrosion depth until 144 hours. Therefore, in the product of the present invention, the average corrosion depth gradually increases with time in a stress corrosion cracking environment, but the maximum corrosion depth changes at a constant corrosion depth. Local corrosion is prevented, and it has been proved that the material is less prone to cracking, which is the origin of stress corrosion cracking.

  In order to quantitatively evaluate the stress corrosion cracking property, the time taken for the alloy to break was compared. The test method is to prepare a test piece as shown in FIG. 46, and sandwich the recesses e on both ends of the test piece with a mounting jig (not shown), and then use a tensioning device (not shown) having a spring with a spring constant of 150 N / mm. A tensile load was applied to maintain the load until breakage, and the time when the breakage occurred in the shaded area in FIG. 46 (a) was measured. This breaking time was measured by photographing a jig installed in a desiccator with a CCD camera and confirming it by video recording. As test conditions, the ammonia concentration was 14%, and the load stress was 50 MPa, 125 MPa, and 200 MPa. As a test piece, the product of the present invention having chemical component values in Table 18 and Comparative Examples 1 and 2 were used. The test results are shown in FIG.

From FIG. 47, the load stresses of 125 MPa and 200 MPa show almost the same fracture time, but at a load stress of 50 MPa, the fracture time of the product of the present invention is longer than those of Comparative Examples 1 and 2, It can be judged that the stress corrosion cracking resistance is improved. At this time, if cracks occur due to corrosion at load stresses of 125 MPa and 200 MPa, the effect of stress is large and the crack progresses and leads to rupture, so that it is considered that there is no difference in material. It is small, and it is thought that the time to crack initiation is greatly affected by the form of corrosion.
In the product of the present invention, the maximum corrosion depth is constant after the corrosion time 24h, and local corrosion is suppressed.

As described above, the product of the present invention has a corrosion state in which the γ phase in the vicinity of the surface layer is uniformly corroded and relaxes stress concentration. it is possible to greatly improve the resistance to stress corrosion cracking if. Further, when the microstructure of the fracture surface after the test was observed, the surface layer of the product of the present invention exhibited uniform corrosion, Comparative Examples 1 and 2 exhibited local corrosion, and were visually resistant to stress corrosion cracking. We were able to confirm superiority or inferiority.

  The brass alloy excellent in stress corrosion cracking resistance of the present invention has not only stress corrosion cracking resistance but also machinability, mechanical properties (tensile strength, elongation), dezincing resistance, erosion / corrosion resistance, and casting resistance. The present invention can be widely applied to all fields where crackability and impact resistance are also required. In addition, an ingot is manufactured using the brass alloy of the present invention, and this is provided as an intermediate product, or the alloy of the present invention is processed and formed into a wetted part, a building material, an electrical / mechanical part, Ship parts, hot water-related equipment, etc. can be provided.

  The members / parts suitable for the brass alloy having excellent stress corrosion cracking resistance of the present invention are water contact parts such as valves and faucets, that is, ball valves, hollow balls for ball valves, butterfly valves, gates. Valves, globe valves, check valves, valve stems, faucets, water heaters, fittings for hot water flush toilet seats, water supply pipes, connection pipes and fittings, refrigerant pipes, electric water heater parts (casing, gas nozzle, pump parts, Burners, etc.), strainers, water meter parts, submersible sewer parts, drainage plugs, elbow pipes, bellows, toilet flanges, spindles, joints, headers, branch plugs, hose nipples, faucet fittings, stopcocks, Water supply and drainage faucet supplies, sanitary ware fittings, shower hose fittings, gas appliances, building materials such as doors and knobs, home appliances, Saya tube headers Use adapter, automotive cooler parts, fishing parts, microscope parts, water meter parts, meter parts, can be widely applied to railway pantograph components, other components and parts. Furthermore, toilet articles, kitchen articles, bathroom articles, toilet articles, furniture parts, living room articles, sprinkler parts, door parts, gate parts, vending machine parts, washing machine parts, air conditioner parts, gas welder parts Widely used in parts for heat exchangers, solar water heater parts, molds and parts, bearings, gears, parts for construction machinery, parts for railway vehicles, parts for transportation equipment, materials, intermediate products, final products and assemblies Applicable.

Claims (2)

In terms of mass ratio, Cu 59.5 to 66.0%, Sn 0.7 to 2.5%, Bi 0.5 to 2.5%, Sb 0.05 to 0.6%, the balance being α + γ containing Zn and inevitable impurities A brass alloy having a structure or an α + β + γ structure, wherein Sb in the brass alloy component is dissolved in the γ phase, and γ for each crystal grain when the γ phase in the brass alloy surrounds each crystal grain The proportion of phases is defined as the γ-phase grain enclosure ratio, and the γ-phase average grain enclosure ratio, which is the average value of the γ-phase grain enclosure ratio, is set to 28% or more, so that the rate of progress of corrosion cracking in the brass alloy is increased. Lead-free brass alloy with excellent stress corrosion cracking resistance, characterized by being suppressed and improved stress corrosion cracking resistance. The lead-less brass alloy excellent in stress corrosion cracking resistance according to claim 1, which contains Se: 0.01 to 0.20 mass% .

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